1. Field of the Invention
The present invention relates to a disk drive apparatus equipped with a plurality of disks on which servo information is prerecorded, and more particularly to a disk drive apparatus and method suitable for compensating for non-continuity of servo information that occurs, for example, when one head is selected.
2. Description of the Related Art
A hard disk drive is known as a disk drive using disks as storage mediums. In a hard disk, in general, a head is moved to a target track by seek control and positioned in a target area on the target track, on the basis of servo information prerecorded on each disk.
In the prior art, servo information is written to each disk in the following procedure, after heads and disks are assembled into a head/disk assembly (hereinafter referred to as an “HDA”). First, the HDA is set in a servo track writer (STW). Subsequently, under the control of the servo track writer, the heads in the HDA write servo information to the respective disks therein. This servo information writing method is, however, inefficient.
Recent servo track writers are configured to simultaneously write servo information to the disks of a plurality of drives. To enable this writing, a plurality of disks are stacked in a servo track writer of this type. Further, heads dedicated to the writing of servo information are provided at the respective positions of the disks stacked in the servo track writer. These heads simultaneously write servo information to the respective disks. This servo information writing method (hereinafter referred to as a “pre-servo method”) enhances the efficiency of writing servo information to the disks.
The hard disk drives include an IC (Integrated Circuit), called a read/write (R/W) channel, i.e. a read/write IC. The read/write IC executes various types of signal processing, such as an A/D (Analog/Digital) conversion process on a read signal read from a disk, an encoding process on write data, and a decoding process on read data, etc. The read/write IC also executes a servo information detection (reading) process. In general, the servo information contains various types of data, such as a servo mark, a servo sector address, a cylinder address (cylinder code) and burst data, etc. Accordingly, in the prior art, it is necessary to supply various timing signals to the read/write IC for servo information detection (reading).
On the other hand, a recent read/write IC, which is multifunctional owing to the developed IC techniques, can generate all timing signals necessary for the detection of servo information. This read/write IC can generate the timing signals in synchronism with a servo gate (a servo gate signal).
The method of enabling the detection of the servo information only utilizing a servo gate is called a synchronous servo method. In a hard disk drive that uses the synchronous servo method, a servo gate is generated by a servo controller. The servo controller is generally formed of an ASIC (Application Specific Integrated Circuit).
As described above, in the pre-servo method, in which the disks of a plurality of drives are stacked in the servo track writer, thereby simultaneously writing servo information to the disks, the efficiency of writing servo information to each disk is enhanced. However, where a number of required disks with servo information are assembled into an HDA, and this HDA is mounted in a hard disk drive, it is possible that servo-information-recorded positions (hereinafter referred to as “servo positions”) on one disk may be physically displaced, circumferentially or radially, from those on another disk.
On the other hand, in a hard disk drive that uses the synchronous servo method, it is necessary to synchronize the leading edge of each pulse of a servo gate with each servo position. However, where the servo positions on one disk are physically displaced from those on another disk as a result of the use of the pre-servo method, the timing of a servo gate differs from an actual servo position upon, for example, head selection.
Thus, where the timing of a servo gate differs from an actual servo position, the seek time required to move the head to a target track on the basis of servo information is increased. Furthermore, a servo mark, as servo identification information contained in the servo information, may not be detected (servo mark absence), or the servo information may be detected at an erroneous timing (erroneous servo detection), which causes an erroneous output timing of a servo gate and hence a reduction of the seek performance.